(1) Technical Field
The present invention relates to solid-state electronic devices. More specifically, the present invention relates to an improved heterojunction bipolar transistor.
(2) Discussion
The transistor is an important example of an “active” component, a device that can amplify an incoming signal to produce an output signal with more power than the input signal. The additional power comes from an external power source.
Two important classes of transistors are field-effect transistors and bipolar junction transistors (BJTs). Broadly speaking both of these classes of transistors are charge-control devices. However, in an npn BJT, the collector-base junction is back-biased so that no current normally flows. Forward-biasing the base-emitter junction causes electrons to enter the base region, where they are attracted to the collector, resulting in a collector current controlled by the base current. The collector current is proportional to the injection rate of minority carriers into the base. This injection rate is an exponential function of the base-emitter potential difference.
An important class of bipolar junction transistors is that of heterojunction bipolar transistors (HBTs). HBTs have been fabricated lattice-matched to GaAs and InP substrates. The applied voltage needed to initiate current flow across the emitter-base junction, Vbe, in these devices ranges from 0.65 to 1.5V, and is an important consideration in device power requirements. There exists a need in the art for a device that exhibits a decreased Vbe (<0.65V), to enable fabrication of devices such as low supply voltage integrated circuits (ICs) that will consume substantially less power than existing HBT ICs.
There have been many publications related to HBT technologies. Citations to a few recent examples are presented below as further resources for the reader:
(1) Q. Lee et al., IEEE Electron Device Letters, Vol. 19, p. 77 (1998);
(2) P. J. Zampardi et al., IEEE Electron Device Letters, Vol. 17, p. 470 (1996);
(3) Y. Matsuoka et al., J. AppI. Phys., Vol. 35, p. 5646 (1996); (4) D. L. Miller et al., J. Vac. Sci. Technol. B, Vol. 16, p. 1361 (1998); and
Pekarik et al., J. Vac. Sci. Technol. B, Vol. 10, p. 1032 (1992)—this particular reference discusses a low Vbe pnp HBT that has been fabricated from 6.1 Å materials; however, npn HBTs are generally better for high-speed applications because electron mobilities are much greater than hole mobilities (electrons carry the current in a npn device, while holes carry the current in a pnp device).